95% titanium dioxide glass spheroids

ABSTRACT

DISCLOSED IS THE PRODUCTION OF GLASS HAVING AT LEAST A 75% BY WEIGHT EQUIVALENT CONTENT OF TITANIUM DIOXIDE. THE GLASS AS SMALL SPHERE IS USEFUL AS RETROREFFECTIVE LENS FOR TRAFFIC MARKINGS SURFACES AND OTHER USES, WHEREIN THE GLASS SPHERICAL ELEMENTS OR BEADS CONTAIN GREATER THAN 75% BY WEIGHT OF TITANIUM DIOXIDE AND ARE SUITABLE AS RETROREFLECTIVE LENS ELEMENTS. DISCLOSED IS THE PRODUCTION OF GLASS HAVING AT LEAST A 75% BY WEIGHT EQUIVALENT CONTENT OF TITANIUM DIOXIDE. THE GLASS AS SMALL SPHERES IS USEFUL AS RETROREFLECTIVE LENS FOR TRAFFIC MARKING SURFACES AND OTHER USES, WHEREIN THE GLASS SPHERICAL ELEMENTS OR BEADS CONTAIN GREATER THAN 75% BY WEIGHT OF TITANIUM DIOXIDE AND ARE SUITABLE AS RETROREFLECTIVE LENS ELEMENTS.

Feb. 2, 1 971 c. E. SEARIGHT ETAL 3,560,074

9570 TITANIUM DIOXIDE GLASS SPHEROIDS Original Filed Oct. 23, 1964 POWERSUPPLY IIIIIIIIIIIIIII.vlllllllllllllllllllllfl //VVE/V7'0R5- CHARLES E.SEAR/6H7 EZRA M. ALEXANDER JOHN R. RYAN STEVEN H. BRASF/ELD A TTORNEYSUnited States Patent 3,560,074 95% TITANIUM DIOXIDE GLASS SPHEROIDSCharles E. Searight, Ezra M. Alexander, John R. Ryan, and Steven H.Brasfield, Jackson, Miss., assignors to gitaphote Corporation, Jackson,Miss., a corporation of 10 Continuation of application Ser. No. 405,925,Oct. 23, 1964. This application Oct. 21, 1968, Ser. No. 769,433 Int. Cl.G02b 5/12 US. Cl. 350-105 5 Claims ABSTRACT OF THE DISCLOSURE Disclosedis the production of glass having at least a 75% by weight equivalentcontent of titanium dioxide. The glass as small spheres is useful asretroreflective lens for traffic marking surfaces and other uses,wherein the glass spherical elements or beads contain greater than 75%by weight of titanium dioxide and are suitable as retroreflective lenselements.

This application is a continuation of our copending application Ser. No.405,925, filed Oct. 23, 1964, now abandoned.

This invention relates to the production of glass, particularly smallglass spherical elements or beads used as lens elements for focusing andreturning incident light to its source.

Small glass beads are used as retroreflective optical elements toretract light rays from a source, such as headlamps of an automobiledriven by an observer, to an average focal point on the face of the beadadjacent a reflector, such as a sign, to concentrate the light. Theconcentrated light is then reflected back through the glass head so thatthe majority of the brilliant beam of reflected light returns to itssource. These is some coning of the reflexed light beam such that anobserver, at a slightly different angle from the light source, observesthe reflexed light from all the glass beaded surface of a sign as abrilliant illumination.

The sizes of the glass beads or spheres used as retroreflective opticalelements range from a fraction of a micron to higher sizes such as up to0.25 inch diameter. Normally, however, the glass beads are not largerthan 2 mm. in diameter and generally are much smaller. In use, the beadsare often bonded to one face of a reflecting surface with the other faceof the beads exposed to light rays.

A wide variety of glass compositions has been used in the past to makeretroreflective glass beads. Most of the glasses, however, have muchlower refractive indices than are desired in beads used asretroreflective optical elements. While glasses of higher refractiveindices can be produced, they are generally expensive and therefore whatis needed is a relatively inexpensive easily made glass bead having ahigh refractive index.

Although many glasses employ titanium dioxide as an ingredient becauseit has good optical properties in glasses, particularly because it has ahigh index of refraction (namely of the order of 2.9 with respect to thed-line of sodium for the rutile form), no glasses have been apparentlyproduced in which the content of titanium dioxide is greater than 75% byweight. It has not been possible to make glasses that have more than 75%by weight of titanium dioxide by the prior art procedures of furnace andrefractory technology since the compositions could not be melted, finedand vitrified to produce the desired glasses. Furthermore, even inproducing glasses up to 75% by weight of titanium dioxide, one or morefluxing agents have been needed to melt, fine and 3,560,074 PatentedFeb. 2, 1971 ice form vitreous glasses containing such amounts oftitanium dioxide using conventional prior art furnace and refractorytechnology.

It has been discovered according to the present invention that beads orspheroidal lens elements which contain the chemical equivalent of morethan by weight of titanium dioxide can be produced by subjecting smallsolid particles of titanium dioxide to sutficient heat to melt theparticles and thereby cause them to become molten spheroids and thencooling the resulting spheroids into solid beads. The beads are suitableas retroreflective lens elements.

To melt particles which are more than 75% by weight of titanium dioxiderequires the utilization of temperatures substantially higher than areused in conventional glass technology.

Various means may be employed to heat the solid particles of titaniumdioxide sufliciently high to cause them to melt. Thus, there can be usedthe flame produced from an acetylene torch, the so-called combex-ADLburner which utilizes a low current, high voltage AC electricaldischarge superimposed on the combustion energy of a hydrocarbon fueland oxygen mixture, or the heat may be obtained by using a plasma flameor even an electrical arc can be used. However, a plasma flame isparticularly useful.

A plasma flame is one comprising plasma, i.e., material which is inenergy state above that which exists in the gaseous state, being furthercharacterized in that at least some of the atoms of material have beendepleted of some of the electrons which they normally possess in thefree state. Plasma flame temperatures up to 60,000 F. can be readilyobtained. Although the invention can utilize a plasma flame, it shouldbe understood that the method or means used to generate such a flamedoes not per se constitute part of the invention. Suitable devices forproducing a plasma flame jet are known. As an example, U.S. Pat. No.2,960,594, which issued on Nov. 15,

1960, to M. L. Thorpe, describes both the theoretical andv practicalaspects of suitable plasma flame generators. Equipment satisfactory foruse in the present invention is commercially available.

The invention will now 'be described further in conjunction with theaccompanying drawing in which:

FIG. 1 depicts schematically a typical embodiment of apparatus which canbe used in carrying out the method of the invention; and

FIG. 2 depicts a typical retroreflective device in which the beads oftitanium dioxide made in accordance with the method of the invention canbe used.

The apparatus depicted in FIG. 1 comprises a hopper 10 containing a massof titanium dioxide particles. The hopper has port 11 communicatingtherewith from which the titanium dioxide particles can freely flowdownwardly as a stream 12 of particles under the influence of gravity.As the stream 12 falls, it passes transversely across the orifice 14 ofa plasmajet generator 26 which is provided with suitable power supply 27and gas supply 28 for producing a plasma flame.

Under the influence of the plasma flame emerging from the orifice 14 ofthe generator 26, the stream of solid titanium dioxide particles isheated, melted and spheroidized into spheres 17 of molten glasseoustitanium dioxide which cool during their free fall to a suitablecontainer 18 where they are collected. The distance through which thesubdivided particles of hot molten titanium dioxide glass are allowed tofall should be suflicient to permit the molten spheres to become solidbefore striking any object which might tend to distort them.Alternatively, a stream of cooling air 30 can be caused to pass throughthe falling particles in order to hasten solidification.

Another suitable method (not illustrated) for feeding the particles oftitanium dioxide into the plasma involves the use of a powder feedsystem of a type known to those skilled in the art. In such a system, asupply of particles of the appropriate size are charged to a hopper,from which they are fed by means of a carrier gas under low pressureinto the center of the stream of plasma. The carrier gas is usuallynitrogen, although other gases can also be used.

The temperature of the plasma flame used must be high enough to melt thetitanium dioxide particles. Useful temperatures are typically in therange from about 3,500 F. to about 12,500 E, which upper limit isusually sufficient to spheroidize into glass bead particles which arefrom above 75% to 100% by weight titanium dioxide. If necessary,however, the upper limit can readily be increased to temperatures suchas up to 30,000 F. or even higher by suitable selection of the gas usedin the plasma generator and the addition of suflicient electrical energyto achieve this temperature, as will be apparent to those skilled in theart.

It is sometimes desirable to retard heat loss from the plasma flame tofacilitate melting the titanium dioxide particles. One such suitableheat retaining and localizing apparatus is shown in the drawing andcomprises a double walled cylinder 21 through the annular space of whichcooling water is supplied by means of entrance pipe 22 and withdrawnthrough exit pipe 23. The stream of titanium dioxide particles passesthrough the cylinder along its axis while the plasma flame is injectedtangentially into the cylinder thereby creating a downwardly movingvortex. This heating system can be used to produce practically anydesired temperature. The temperature created within cylinder 21 may beso high in fact that the function of the cooling water stream suppliedby pipe 22 is to prevent melting or heat deformation of the material ofwhich the cylinder is made, suitably a metal such as copper.

The gases which are suitable for use in the plasma generator inaccordance with the invention are generally the inert gases such asnitrogen, argon, and helium or various mixtures thereof. The choice ofgas is outside the scope of this invention and a suitable selection willbe apparent to one skilled in the art.

Although particles which are more than 75% by weight of titanium dioxidecan be vitrified according to this invention and useful glasses therebyobtained, such as glass heads, the invention is particularly useful inmaking glasses from commercially available titanium dioxide whether inthe rutile, anatase or brookite form. Commercially available titaniumdioxide is readily obtained of 95% or higher purity, with 98% puritybeing quite common. It is such materials which find great usefulness inthis invention since glasses made therefrom have high indices ofrefraction, with indices of 2.7 or higher being common.

While it is advisable to use particles which are almost entirelytitanium dioxide, there can also be obtained useful glasses in which thetitanium content is considerably less, but above 75% by weight Thus,particles having 75 titanium dioxide fused with a wide variety ofinorganic oxides such as lead oxide, barium oxide, magnesium oxide,silica oxide, calcium oxide, antimony oxide, molybdenum oxide andaluminum oxide and mixtures of these and other oxides can be made intoglasses according to this invention. Simple mixtures of titanium dioxidewith such oxides do not function well in making the glasses. Thetitanium dioxide and one or more of the other oxides should first befused together into a homogeneous mass having more than 75% titaniumdioxide and then crushed into small particles for vitrification.

The particles used in the invention, particularly for making glassbeads, should be such that upon melting and spheroidization, beads areobtained having an average diameter less than 1 mm. (1,000 microns).Better results are obtained when the particles give beads less than 500microns average diameter, and desirably less than 200 microns andadvisably less than microns for substantially pure titanium dioxide.

The particles, once molten, should be maintained free of contact withother molten particles to prevent consolidation thereof and theformation of larger beads. The molten particles should also be kept outof contact with other surfaces until they are solidified to preservetheir spheroidal shape.

Beads produced according to this invention are colorless although ifsome reduction of titanium dioxide occurs they can be colored from atrace of blue to almost black.

The beads provided herewith can be used for all the purposes for whichprior art beads have been employed, including highway marking stripesand signs of all types.

The following examples are presented to illustrate the invention. Ineach case, the product comprised glass particles suitable for use inmaking retroreflective sheeting materials.

EXAMPLE 1 Particles of glass grade titanium dioxide were passed throughthe flame from an acetylene torch, the particles melted, and aftercooling collected as small vitreous beads. They were translucent tovisible light but due to some reduction of the titanium dioxide theircolor ran from blue to black.

EXAMPLE 2 Particles of titanium dioxide (98% purity) were passed througha plasma flame produced using Standard cubic feet of nitrogen or anitrogen-hydrogen mixture per hour with a power consumption of about 40k.w. giving an output of 100,000 B.t.u. per hour and a flame temperatureof about 6,000 P. The particles of titanium dioxide were less than 2 mm.in diameter (after spheroidization by the plasma flame). After themolten spheroids of titanium dioxide were produced they were cooled inair prior to contacting any surfaces. The resulting glass beads of atleast 98% titanium dioxide were vitreous but slightly colored due toreduction of some of the titanium dioxide to the lower valence titaniummonoxide (TiO) thereby giving a blue color to the beads.

EXAMPLE 3 An apparatus was arranged as shown in the drawing attachedhereto using a 4 inch diameter copper tube of high strength to form theradiant wall into which the plasma flame was projected and through whichtitanium dioxide particles were fed. The pipe was 18 inches in length.Appropriate water cooling was provided by enclosing the tube in a largerpipe and sealing off the space between the two pipes as shown in thedrawing and also by providing inlet and exit water pipes.

A plasma flame was employed utilizing nitrogen gas at 15 cu. ft. perhour giving a continuous enthalpy of 60,000 B.t.u./ lb. and a flametemperature of 26,000 F. At about 10% efiiciency for heat transfer theplasma flame was calculated sufficient to spheroidize about 12 lbs. oftitanium dioxide per hour.

Twenty parts of calcium oxide and eighty parts of titanium dioxide weremelted and fused together using a conventional glass furnace andrefractory. After fusion the calcium titanate was withdrawn from thefurnace into a water bath wherein it was cooled. The cooled fusionproduct was then crushed to particle sizes less than 2 mm. averagediameter and then dropped by means of a hopper as shown in the drawingthrough the plasma flame to melt the particles. The particles were thencooled and collected as small solid beads or spheroidal elements. Theresulting calcium titanate beads had an index of refraction no less than2.5. An oxygen purge was used in the collecting vessel in order toprevent reduction of the glass during the bead forming and coolingoperation.

Instead of nitrogen other gas can be used in the plasma, such ashydrogen, argon, helium, neon and krypton.

A typical retroreflective device in which beads made in accordance withthe invention can be used is shown in FIG. 2. As there depicted, thedevice comprises a reflec tive back sheet 41 having attached thereto alayer of glass beads 42 which are held in position by means of asuitable transparent binder 43. Covering the top face of the glass beadsis a suitable transparent film 44 having a fiat top surface. Devices ofthis type are described in US. Pat. No. 2,407,680 issued Sept. 17, 1946.The optimum effective index of refraction for such a device is known tobe about 1.9, which value represents the ratio of the index ofrefraction of the beads to the index of refraction of the transparenttop film. The relatively high index of refraction of the beads made inaccordance with the invention, which can be as high as 2.9 or greater,thereby renders feasible the use of commercially available transparentfilms having excellent physical properties and high transparency, suchas polymethylmethacrylate which has an index of refraction of about 1.5,and other methacrylate polymers having values down to about 1.45.

Various changes and modifications of the invention can be made and, tothe extent that such variations incorporate the spirit of thisinvention, they are intended to be included within the scope of theappended claims.

What is claimed is:

1. Translucent glass spheroids of less than 2 millimeter averagediameter, the ingredients of which contain more than about 95 by weightof titanium dioxide.

2. Transparent glass spheroids of less than 1 millimeter averagediameter, the ingredients of which contain more than 95 by weight oftitanium dioxide.

3. Glass spheroids of less than 1 millimeter average diameter, theingredients of which consist essentially of titanium dioxide.

4. A retroreflective device comprising a sheeting material havingaffixed to at least one surface thereof small glass spheroids whichaverage less than 1 millimeter in diameter, the ingredients of whichglass contain more than about 95% by weight of titanium dioxide and atransparent film having a flat outer face overlying said glassspheroids.

5. A retroreflcctive device according to claim 4, in which theingredients of the glass consist essentially of titanium dioxide.

References Cited UNITED STATES PATENTS 3,493,403 2/ 1970 Tung et al.6521X 2,568,126 9/1951 Keeley. 3,293,051 12/1966 Searight.

S. LEON BASHORE, Primary Examiner J. H. HARMAN, Assistant Examiner US.Cl. X.R.

